This application claims the priority of Korean Patent Application No. 2002-83202 filed Dec. 24, 2002 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to an apparatus and method of calibrating the marking position of a chip scale marker, and more particularly, to an apparatus and method of calibrating the marking position of a chip scale marker which marks a character on a wafer chip using a laser beam.
2. Description of the Related Art
Several thousands or tens of thousands of chips are formed on a wafer used in a semiconductor process. To classify the chips according to production lots, characters and/or numbers are marked on a surface of each chip. A chip scale marker using a laser beam is used as an equipment for marking.
FIG. 1 is a view illustrating a constitution of a typical chip scale marker. FIG. 2 is a view illustrating a constitution of the laser system of FIG. 1.
Referring to FIGS. 1 and 2, a wafer w is placed on a wafer holder 20 and a laser system 30 is arranged under the wafer holder 20. A laser beam oscillated by a laser oscillator 31 of the laser system 30 is magnified by a beam expander 32 and input to a Galvano scanner 33. The laser beam input to the Galvano scanner 33 is irradiated onto chips on the wafer w through an f-θ lens 34 so that characters are recorded on a surface of the chips. The above laser system is disclosed in Japanese Patent Publication No. H9-248692.
A camera 40 monitoring the wafer w being supported by the wafer holder 20 is arranged above the wafer holder 20. The camera 40 is moved by being connected to an X-Y stage 50.
FIG. 3 is a view illustrating that a marking shape is distorted by the Galvano scanner. The Galvano scanner 33 includes an x mirror 33a and a y mirror 33b. The x mirror 33a controls movement of a laser beam in a direction x by an x drive (not shown) rotating a shaft 33c at one end of the x mirror 33a. The y mirror 33b controls movement of a laser beam output from the x mirror 33a in a direction y by a y drive (not shown) rotating a shaft 33d at one end of the y mirror 33b. Thus, an optical path in the direction x is longer than that in the direction y. Accordingly, when a signal to mark a rectangular shape as shown in FIG. 4A is transmitted, a distortion like a pin cushion as shown in FIG. 4B is generated. Also, a positional error is generated due to a tiny difference in position between center lines of rotation shafts 33c and 33d of the x and y mirrors 33a and 33b and surfaces of the x and y mirrors 33a and 33b. 
In the meantime, as the beam having passed through the Galvano scanner 33 passes through the f-θ lens 34, the beam is curved so that a barrel distortion is generated. To improve the distortion phenomenon of the marking, marking calibration should be periodically performed to control the rotation of the x and y mirrors 33a and 33b of the Galvano scanner 33.
FIG. 5 is a view illustrating a conventional method of measuring a marking error. According to the conventional method, a laser beam is irradiated onto a plate 70 having a shape and size corresponding to a wafer and a plurality of small holes 70a having a diameter of 0.3 mm formed in the plate 70 at a predetermined interval, and the position of the laser beam passing through each of the holes 70a is observed by the camera 40 and compared with a target position of the laser beam. Next, a degree of an error in the irradiation position of a laser beam is recognized and a path along which the laser beam is irradiated is corrected.
However, in the conventional method, since the laser beam passing through the holes 70a is observed through a glass portion 42 in front of the camera 40, a laser beam inclined with respect to the holes 70a, as indicated by a dotted line of FIG. 5, is refracted by the glass portion 42 of the camera 40. Thus, an accurate position on the plate 70 where the laser beam is irradiated is difficult to recognize and it takes some time for the camera 40 to move above the holes 70a to be measured. Also, the plate 70 may be damaged by power of the laser beam.